One type of the display devices employing self-luminescence elements utilizes electroluminescence elements (EL elements). The EL element is divided into an organic EL element having a light emitting layer made of an organic material and an inorganic EL element having a light emitting layer made of an inorganic material.
The organic EL element includes an anode, a cathode, and an organic EL layer which is sandwiched between these two types of anode and cathode electrodes and which is made of thin film of an organic light emissive compound. Applying a voltage between the anode and the cathode causes the anode and the cathode to inject holes and electrons into the organic EL layers respectively, for recombination. The energy produced then excites the molecules of the organic light emissive compound constituting the organic EL layer. A light emitting phenomenon is provided in the process of such excited molecules being deactivated into their ground state. The organic EL element is a light emitting element which utilizes this light emitting phenomenon.
The organic EL layer includes at least an organic layer called a light emitting layer in which holes and electrons are recombined to emit light. When necessary, the organic EL layer has a single-layer structure or a multi-layered structure that includes one of or both an organic layer called a hole transport layer which allows holes to be readily injected therein but electrons to hardly travel therethrough and an organic layer called an electron transport layer which allows electrons to be readily injected therein but holes to hardly travel therethrough.
In recent years, the organic EL element is actively being studied and brought to practical use. This element has a basic structure in which a hole injection material such as triphenyldiamine (TPD) is evaporated to form a thin film on a transparent electrode (a hole injection electrode or an anode) such as indium tin oxide (ITO), and a phosphor such as an aluminum quinolinol complex (Alq3) is then deposited as a light emitting layer, with a metal electrode (an electron injection electrode or a cathode) having a low work function such as AgMg being subsequently formed. Attention is now focused on this element because the element provides as very high a brightness as several hundreds to several tens of thousands of cd/m2 at a voltage of about 10V and thus can be used as an illumination lamp, a light source, or a display for OA devices, home electric appliances, automobiles, two-wheeled vehicles, airoraft, and so on.
For example, such an organic EL element is configured such that an organic layer such as a light emitting layer is sandwiched between a scan (common line) electrode serving as an electron injection electrode and a data (segment line) electrode serving as a hole injection electrode (transparent electrode), and is formed on a transparent (glass) substrate. On the other hand, the display is largely divided into two types: a matrix display that allows the light emitting elements disposed in a matrix to emit light dot by dot using the scan electrodes and data electrodes arranged in the horizontal and vertical directions in order to display information such as an image or character as a collection of these dots (pixels), and a segment display that allows an indicator present independently as having a predetermined shape and size to be displayed.
For the segment type display, it is possible to employ a static drive system by which each indictor is displayed separately independently. However, for the matrix display, normally employed is a dynamic drive system which allows each scan line and data line to be driven in time division manner.
The light emitting element constituting the light emitting portion of the organic EL element is divided into the following types: a substrate surface emission type that uses the structure of transparent substrate/transparent electrode/light emitting layer/metal electrode allowing light generated in the light emitting layer to be emitted through the transparent electrode and the transparent substrate, and a film surface emission type that uses the structure of substrate/metal electrode/light emitting layer/transparent electrode allowing light generated in the light emitting layer to be emitted through the transparent electrode from the film surface side opposite to the substrate surface. The element of the substrate surface emission type is described, for example, in Appl. Phys. Lett., 51, 913-915 (1987), while the element of the film surface emission type is described, for example, in Appl. Phys. Lett., 65, 2636-2638 (1994).
An organic fluorescent solid body serving as a material of the light emitting layer in the organic EL element is quite susceptible to deterioration from moisture, oxygen or the like when exposed thereto. An electrode disposed directly or via the electron transport layer on the light emitting layer is also quite susceptible to deterioration in characteristics from oxidation. This causes a prior art organic EL element to suddenly deteriorate in its emission characteristics when it is driven in the air. In particular, the presence of oxygen or moisture around the element raises a problem of accelerating oxidation thereby causing the organic material to be altered in quality, the film to be peeled off, and a dark spot (non-light emitting portion) to grow, resulting in loss of lifetime. Accordingly, to obtain a practical organic EL element or organic EL device, it is necessary to devise the structure of the element to prevent the intrusion of moisture or oxygen into the light emitting layer and the oxidation of its opposite electrode.
To solve the aforementioned problems, it is suggested to seal the organic EL element to prevent it from being exposed to the air. For example, Japanese Patent Laid-Open Publication. No. Hei 5-182759 discloses that an organic EL element is covered with and thereby sealed in a photocurable resin layer resistant to moisture and a substrate secured onto that layer and having a reduced permeability to moisture. On the other hand, Japanese Patent Laid-Open Publication No. Hei 5-41281 discloses that an El element is sealed in an inert liquid having a dehydrating agent such as synthetic Zeolite contained in a fluorocarbon oil. On the other hand, Japanese Patent No. 2800813 discloses a method for providing an organic EL element with a fluorine-based polymer protective layer, disposing outside that layer a sealing portion having a cap structure, and filling the sealing portion with an inert m dium for encapsulation.
It has also been suggested that deterioration is prevented by capturing moisture. For example, Japanese Patent Laid-Open Publication No. Hei 3-4481 discloses that an organic EL element is coated with a moisture capturing layer. In addition, Japanese Patent Laid-Open Publication No. 2000-30871 describes that a moisture capturing material is contained in an insulating layer that fills between the transparent electrodes arranged in a matrix.
However, the application of only the sealing techniques was not sufficient to completely remove the moisture and oxygen present around the element, thereby making it difficult to ensure a sufficiently long emission lifetime. Furthermore, since usage of the prior art sealing may result in an increase in thickness of the display device by the amount of a sealing member, it is desirable to ensure a light emission sustain time without using the sealing, if possible.
Still furthermore, even the structure provided with a moisture capturing layer raises a problem of causing an increase in thickness of a display device by the amount of the layer. In the methods employing the prior art moisture capturing layer and the moisture capturing material, since the moisture capturing layer or the moisture capturing material is not in direct and entir contact with the organic film, it is difficult to provid a sufficient moisture capturing effect to the organic film.